Congress enacted that law - one of the two crown jewels of the civil-rights movement - because blacks were being denied access to the vote through unfair state-imposed tests in the still-segregated South. Passed a century after the Civil War ended, the law represented a sincere attempt by Congress to make the 15th Amendment's right to vote actually meaningful.

By striking down that law as an outmoded infringement on states' rights, the court has flipped the rules once and for all: The justices, and not the elected Congress, now decide what remedy is needed to effectuate the most basic right in a democracy.

Chief Justice John Roberts explained that, in 1965 and beyond, it was perfectly appropriate for Congress to insist that counties with voter registration tests and voter registration below 50 percent be subjected to special rules. Originally enacted for five years, the VRA was extended repeatedly. Its remedy was that any change to election districts or voting rules in state laws would have to be "pre-cleared" by the Department of Justice before being allowed.

However, almost 50 years later, Roberts wrote, this method of designating what locations would be covered by the VRA has become unconstitutional. What has changed, Roberts explained, is that old-fashioned discriminatory tests are now a thing of the past - in large part, he acknowledged, because of the success of the VRA. As a result, the most recent extension in 2006, for 25 more years, violated the states' inherent rights to "dignity" and "equal sovereignty." The affront, in this view, consists of one state having to wait for the Department of Justice to approve changes in its laws, while other states can proceed apace.

This is a strange conception of why the VRA should be struck down. Certainly the Constitution must protect the existence of the states within the federal system - but a state's capacity to change its districting quickly hardly seems to lie at the core of its continued vitality. Redistricting is everywhere a politicized, ugly business, in which state legislatures game local boundaries and borders to shape results that will serve some political parties and actors while disadvantaging others. Is the right for states to gerrymander quickly really more essential than the right of every individual citizen to vote?

The truth is that the conservative majority sees itself as capable of judging when the wheel of history has turned. A law designed to address a century of denying voting rights to blacks, enacted when Barack Obama was a toddler, looks different during his second term as president.

Discrimination today is subtler, more sophisticated and broader-based. It isn't restricted to the counties of the Deep South. Just ask Latinos in Arizona, where a law recently struck down by the justices would have imposed the extra-legal obligation to provide proof of citizenship when registering by mail.

From the court's perspective, these changes ought to be reflected in constitutional judgment. This means that if Congress simply renews old laws with minor changes, it hasn't kept up with the times. Ordinarily, we would think that the legislature is the right institution to keep our system up-to-date. It's not as though the VRA that was struck down is actually 50 years old; it's more like seven years old, dating to the renewal in 2006. But when it comes to many big issues - race, sex and sexual orientation - the Supreme Court has come to see itself as in the driver's seat, supervising Congress and elected legislatures. But if the court sees itself as a sort of inspector of democracy, it follows (more or less) that it can and should intervene when Congress is addressing the right to vote.

Among liberals, there will be the gnashing of teeth appropriate to such an important historical moment. In her dissent for the four liberals, Justice Ruth Bader Ginsburg cited eight examples of relatively recent discriminatory efforts by covered jurisdictions. She pointed out the absurdity of thinking that the covered jurisdictions aren't especially vulnerable to racist voting discrimination.

But Ginsburg's chosen metaphor compared the danger of racism in those locales to the danger of earthquake in California, thus missing the core of the majority's view. Race may be the San Andreas fault in American life, but racism is a product of culture, not nature - and so it can change and alter in real, observable time, not with the geological ages. In this instance, the conservative majority thought, the Constitution must accommodate recent historical change.

When it comes to the term's two same-sex marriage cases, the situation will be reversed: The liberals will want the court to interpret history to override legislation, and the conservatives will wail about judicial activism. But the truth of our constitutional moment is that both sides believe deep down that the court should interpret the Constitution aggressively in light of the historical moment we now occupy. Liberals and conservatives aren't really arguing about the function of the Supreme Court: They are arguing about the true meaning of our historical moment today. ■